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Homework Help: Heat Transfer Through a Plane Wall

  1. Sep 16, 2011 #1
    1. The problem statement, all variables and given/known data
    Consider a large plane wall of thickness L = 0.4m, thermal conductivity k = 1.8W/(m*K), and surface area A = 30m2. The left side of the wall is maintained at a constant temperature of T1 = 90 C while the right side looses heat by convection to the surrounding air at Ts = 25 C with a heat transfer coefficient of h = 24 W(m2*K). Assuming constant thermal conductivity and no heat generation in the wall evaluate the rate of heat transfer through the wall.
    Answer: 7079 W

    My answer doesn't match the book's answer.

    2. Relevant equations
    [tex]\dot{Q}_{wall} = -kA\frac{dT(0)}{dx}[/tex]
    [tex]\frac{d^2T}{dx^2} = 0[/tex]
    [tex]T(0) = 90[/tex]
    [tex]-k\frac{dT(L)}{dx} = h[T(L) - Ts][/tex]

    3. The attempt at a solution

    Solving the differential equation and applying B.C.:
    [tex]T(x) = xC_1 + C_2[/tex]
    [tex]T(0) = C_2 = 90[/tex]
    [tex]-kC_1 = hLC_1+hC_2-25h][/tex]
    [tex]C_1 = -\frac{h(C_2-25)}{k+hL}[/tex]

    Plugging numbers in:
    [tex]C_2 = 90[/tex]
    [tex]C_1 = -136.8[/tex]
    [tex]T(x) = 90-136.8x[/tex]
    [tex]\dot{Q}_{wall} = -1.8*30*(-136.8) = 7,387 W[/tex]

    Did I make a mistake or is the book's answer wrong?
  2. jcsd
  3. Sep 17, 2011 #2

    rude man

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    Gold Member

    I used a somewhat different approach (underlying equations are of course the same) and got the same answer you did.

    I computed the thermal resitance of the wall as L/kA = 7.41e-3 K/W and the equivalent thermal resistance of the convection effect as 1/hA = 1.39e-3 K/W, giving total thermal resistance of 8.80e-3 K/W so dQ/dt = (90 - 25)/8.80e-3 = 7387W.

    Just curious - what is your textbook?
  4. Sep 17, 2011 #3
    Awesome thanks, I didn't want to waste any more time redoing this problem looking for a mistake, and my class is just starting the thermal resistance chapter so if I see more problems that have wrong answers I'll do them that way to check.

    We're using Heat and Mass Transfer Fundamentals & Applications 4th Ed by Cengel and Ghajar.
  5. Oct 18, 2011 #4
    arent we suppose to take temp. as kelvin here?
  6. Oct 18, 2011 #5

    rude man

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    It's OK to interchange K and C as long as you're dealing with temperature differences.
  7. Oct 18, 2011 #6
    oh ure right. new to the subject, just trying to learn sorry :) btw I personally know professor cengel and i will inform him about this. I will let you know about it. if you suspect any other mistake in the book feel free to ask please.
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